Many decades of experience have permitted development of highly reliable, sturdy, and inexpensive postage meters (franking machines). These include the many models of postage meter developed by the same assignee as the assignee of the present invention, such as the Smile series of meters.
Notwithstanding the high reliability, sturdiness, and inexpensiveness of such postage meters, some postal authorities have suggested that such meters be discontinued from use and that postage indicia be printed instead by means of digitally controlled printers, typically employing off-the-shelf inks and printing devices. While at first blush it might seem that such printing methods would be subject to fraud of many different types, it has been suggested that this great risk of fraud might be protected against by placing a machine-readable entity such as a high-resolution two-dimensional bar code in the digitally printed indicium. The bar code would contain information that has been cryptographically processed to permit authentication of the bar code, and thus of the legitimacy of the postal indicium.
A critical service decision of the postal authority is what to do about mail pieces that have postal indicia for which the bar code cannot be read completely enough to permit its authentication. One choice is to mark all such mail "return to sender", a choice which would be extremely undesirable if the result were the return of an appreciable number of mail pieces which were, in fact, authentic despite having bar codes that, for some reason, cannot be read in sufficient detail to permit authentication.
The other possibility is to deliver mail pieces for which the bar code is not sufficiently readable, rather than to return them to the sender. If such a policy were adopted and if it were to become generally known, then many parties would be tempted to attempt to obtain free postal services by using conventional printers to print plausible-looking postage indicia that contained bar codes with intentionally unreadable bar codes.
In the face of these concerns, it is immediately apparent to those skilled in the art that the bar code must indeed be of high resolution, with very little dimensional distortion. For the designer of a postage printer, however, this is not an easy goal to achieve. Even the slightest inaccuracy in the measured position (and velocity) of a mail piece will result in a printed indicium that is stretched, or compressed, or otherwise distorted in the dimension through which the mail piece moves during printing.
Those who are skilled in the art of postage printing will immediately appreciate that printing on mail pieces is more difficult than other printing tasks, for example because mail pieces are of varying thickness, are made of varying materials, and each piece tends to be irregular in thickness. It is, by comparison, quite easy to print on sheets of uniform thickness, dimensions, and materials. A further difficulty with printing on mail pieces is that they often move with nonuniform velocity and are introduced into the postage printer at irregular and unpredictable times.
A typical prior art way of measuring movement and position of a mail piece is by means of opposing resilient rollers in gripping contact with the mail piece; a resolver or other position transducer generates a signal indicative of movement of the mail piece. Such a measurement method has numerous drawbacks. For example, the pressured contact of the rollers with the mail piece can cause the mail piece to flex or otherwise move relative to the paper path, thus disturbing its position relative to the print head which is typically an ink-jet print head. Because an ink-jet print head requires precise positioning relative to its target (here, a mail piece), then the flexing and other movement caused by opposed rollers may disturb the printing.
Many other problems present themselves with contact-type movement sensing. For example, a mail piece of irregular thickness will give rise to inaccuracies in the measured movement, because the rollers ride up and down the irregularities of the mail piece.
For all these reasons, it is desirable to provide a reliable means of measurement of movement of a mail piece, preferably a means that does not require contact with the mail piece.